1. Field of the Invention
The present invention relates to optical fiber sensing systems and. more specifically, to a method and system of rendering the shape of a multi-core optical fiber cr multi-fiber bundle in three-dimensional space in real time based on measured fiber strain data.
2. Description of the Background
There are many diverse engineering applications where determining or monitoring the shape of an article or structure is of paramount importance. The science of determining changes to the material and/or geometric properties of a structure is referred to as Structural Health Monitoring (SHM). Generally, SHM involves the observation of a structure over time using periodically sampled measurements from an array of sensors, and the analysis of these measurements to determine the current state of structural health. There are many different sensors and sensing networks for accomplishing this, but many have inherent limitations that render them unsuitable in certain applications. For example, in the field of aeronautics traditional structural health-monitoring of aircraft wings typically involves the use of photogrammetry. In photogrammetry, strategic portions of the aircraft are marked beforehand and a baseline photograph is taken and calibrated to determine the initial displacement. When the wing is under deflection through testing, it is monitored by comparing pre-deflection and post-deflection photographs. This technique requires a clear line of sight, and oftentimes the use of any direct line-of-sight monitoring is either impossible or impractical. Thus, while suitable as for a controlled testing environment the technique of photogrammetry is impractical to monitor aircraft wing-shape in actual flight due to changes in lighting (night-time versus daytime). The same rationale holds for bridges, other concrete structures, and most any solid structure where there is no line of sight through the structure itself. Other techniques include wired networks of strain or temperature sensors, accelerometers, or the like which entails complex wiring layouts. These are costly and impractical, and indeed the wires tend to corrode with age.
Another technique that is rapidly gaining in popularity involves fiber optic sensing networks. See. e.g., Tennyson, “Monitoring Bridge Structures Using Long Gage-Length Fiber Optic Sensors”. Caltrans Bridge Research Conference (2005). Optical fiber sensors typically involve a light propagating beam which travels along an optical filler network. Within each fiber the light is modulated as a function of strain, temperature, bending or other physical or chemical stimuli. The modulation can be analyzed in cither reflection or transmission to determine the characteristic of interest. Optical fiber sensors (OFS) have many distinct advantages including immunity to electromagnetic interference, long lifetime, lightweight, small size, low cost, high sensitivity, etc. Serially multiplexed or branched OFS networks are especially suitable for SHM of large and/or distributed structures which usually need a lot of measurement points.
Sensing the shape of an optical fiber is useful in many applications ranging for example, from manufacturing and construction to medicine and aerospace. In most of these applications, the shape sensing system mast be able to accurately determine the position of the fiber, e.g., within less than one percent of its length, and in many cases, less than one tenth of one percent of its length. There are a number of approaches to the shape measurement problem, but none adequately addresses the requirements of most applications because they are too slow, do not approach the required accuracies, do not function in the presence of tight bends, or fail to adequately account for twist of the fiber (the presence of torsional forces that twist the fiber undermine the accuracy, and thus, usefulness of these approaches).
Conventional approaches to measuring the shape of a fiber use strain as the fundamental measurement signal. Strain is a ratio of the change in length of a fiber segment post-stress verses the original length of that segment (pre-stress). As an object like a fiber is bent, material on the outside of the bend is elongated, while the material on the inside of the bend is compressed. Knowing these changes in local strain and knowing the original position of the object, an approximation of the new position of the fiber can be made. There are even efforts to account for torsional forces.
For example, U.S. Pat. No. 7,813,599 to Jason Moore issued Oct. 12, 2010 discloses a method of determining the shape of an unbounded optical fiber by collecting strain data along a length of the fiber, calculating curvature and bending direction data of the fiber using the strain data, curve-fitting the curvature and bending direction data to derive curvature and bending direction functions, calculating a torsion function using the bending direction function, and determining the 3D shape from the curvature, bending direction, and torsion functions. An apparatus for determining the 3D shape of the fiber inclines a fiber optic cable unbound with respect to a protective sleeve, strain sensors positioned along the cable, and a controller in communication with the sensors. The controller has an algorithm for determining a 3D shape and end position of the fiber by calculating a set of curvature and bending direction data, deriving curvature, bending, and torsion functions, and solving Frenet-Serret equations using these functions.
In order to effectively sense position with high accuracy, several key factors must be addressed. First, for a strain-based approach, the strain measurements are preferably accurate to tens of nanostrain (10 parts per billion) levels. Such high accuracy strain measurements are not readily attainable by conventional resistive or optical strain gauges. Therefore, a new technique that is not strain-based in the conventional sense is required.
Second, the presence of twist in the optical fiber must be measured to a high degree of accuracy and accounted for in the shape computation. The problem is how to obtain an accuracy of rotational position belter than 1 degree. For a high accuracy rotational sensor, the position of strain sensors along the length of the fiber must also be known to a high degree of accuracy. Therefore, some high-accuracy way of measuring the rotation of the fiber is needed to coned the calculation at the fiber position
Third, the prior art shape sensing fiber networks such as shown in U.S. application Ser. No. 12/874,901 by Froggatt el al require optical fibers having multiple fiber cores that are helixed at a known rate with Bragg grating sensors (FBGs) (a conventional optical strain gauge). These fibers arc extremely difficult and expensive to make.
What is needed is a system and method for optical fiber shape that is capable of achieving nanostrain resolutions and including a high-accuracy measurement of the rotation of the fiber in order to correct the calculation of the fiber position. A system and method with such features would have great utility in traditional SHM systems for most any engineering structures, and would find ready application in non-traditional shape sensing applications such as medical tools (e.g., flexible endoscopes and other minimally invasive surgical instruments) or other systems for monitoring and inspection.